1. Field of the Invention
The present invention relates to a binder for heat-sensitive recording materials and, more specifically, to improvements in the water resistance of a coating layer of said recording materials and the storage stability of an undeveloped sheet portion.
2. Prior Art
Heat-sensitive recording materials are now enjoying wide use for output recording of facsimiles, electronic computers, automated ticket dispensers, medical examinations and the like as well as hard copying of CRT, partly because they can develope a clear image by heating and partly because they are advantageous in that the recorders used are relatively of a compact size and maintenance-free, the sheet substrate applied are usually paper which may be more inexpensive than other recording materials (e.g., dielectric recording sheets). Such heat-sensitive recording materials are obtained by applying onto a sheet substrate such as paper, a film or synthetic paper an aqueous solution or dispersion composed mainly of a colorless or light-colored leuco dye, a developer for allowing said dye to develope a color by heating and a binder, followed by drying. As the binders used to this end, it has been proposed to make use of water-soluble high-molecular compounds such as, for instance, polyvinyl alcohol or modified products, starch, or modified products and its derivatives, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose, gelatin, casein, polyvinyl pyrrolidone, polyacrylamides, polyacrylates, styrene/maleic anhydride copolymers and diisobutylene/maleic anhydride copolymers, optionally with a waterproofing agent (formalin, glyoxal, chrome alum, glutar aldehyde, epoxy resins and melamine/formalin resins). It has also been proposed to use a hydrohobic resin, and high-molecular latices or emulsions of styrene/butadiene copolymers, polyvinyl acetate and polyacrylates ester. Whether they are water-soluble or added thereto with the waterproofing agent, however, such binders are poor in water resistance, and are troublesome in handling of said heat-sensitive recording sheets. For instance, when the heat-sensitive recording material is wetted by droplets of water or for some reasons, the coated layer may become sticky, or may peel from the sheet substrate. In addition, after recording, there may be a drop of the developed image density. In some cases, it may be impossible to make out the developed image. Use of the hydrohobic resin is disadvantageous in lowering of heat sensitivity and residue build-up. Some of the waterproofing agent with the water-soluble resin may cause the coagulation of the coating color which brings the coating trouble. Alternatively, there is a fear that ordinary low-temperature drying may lead to insufficient waterproofing (since drying at a temperature of higher than 100.degree. C. may possibly incur color development, drying is usually carried out at a temperature not exceeding 100.degree. C.). In view of the aforesaid considerations, attempts have been made to afford waterproofness to recording materials by various methods. However, none of these methods gives any satisfactory waterproofness. Even when the resulting materials have satisfactory waterproofness, they are found to have other shortcomings. For instance, the compound which is used as the waterproofing agent for the aforesaid water-soluble binders, and has in its molecule at least two 1,2-epoxy ring structures (Japanese Publication Patent No. 51-29947) or at least two ethylenimine groups (Japanese Laid-Open Patent No. 49-32646) gives rise to a lowering of heat sensitivity, which results in a drop of the developed image density and a drop of the stability of the coating liquid due to its coagulation or increases of viscosity, and brings about deteriorations in the storage stability of the heat-sensitive recording materials.
Japanese Laid-Open Patent Nos. 52-73047 and 54-1040 disclose an isobutylene/maleic anhydride copolymer, and Japanese Laid-Open Patent Nos. 54-80136 and 58-89397 teaches use of hydroxyethylcellulose and use of methylcellulose or carboxymethylcellulose with an isobutylene/maleic anhydride copolymer or its water-soluble salt, respectively. Although these compounds are found to give sufficient waterproofness to coated layer, they do not give any satisfactory brightness (of higher than 75%) to products which are formed into heat-sensitive recording sheets. Further, the brightness drops (to 70% or less) due to atmospheric moisture with the lapse of time, thus leading to deteriorations in the aesthetic appearance of the products to be used as recording sheets. Still further, when bar code pattern is recorded, there is a serious problem in which scaning becomes impossible.